Free span assessment and VIV fatigue
Where a pipeline bridges a seabed depression, current and waves shed vortices that can make the span vibrate, and every cycle adds fatigue damage at the weld. The assessment finds the allowable span length and whether each surveyed span can stay as it is or needs support.
When you need it
- A survey has found free spans longer than the design allowed, or new spans from scour.
- Allowable span lengths are needed for a new route before installation.
- Operating conditions change the effective axial force, content density or design life.
- A span sits near a crossing, a structure or an uneven seabed where end conditions are uncertain.
What the analysis covers
- Natural frequencies and mode shapes of the span, with effective axial force and pipe-soil support at the shoulders.
- In-line and cross-flow VIV response from current and waves, using reduced velocity and stability parameter.
- Direct wave loading on the span and the long-term distribution of sea states.
- Fatigue damage at girth welds with the S-N curves of DNV-RP-C203.
- Ultimate limit state checks for combined bending, pressure and axial force.
What we need from you
- Pipe, coating and content data, including concrete coating and marine growth allowance.
- Span survey: length, gap to seabed and shoulder geometry, or the route bathymetry.
- Current and wave long-term distributions, with directions.
- Soil data for the span shoulders: type and stiffness parameters.
- Design life, effective axial force or lay tension, and the weld details for S-N selection.
What you receive
- A report with assumptions, method, verification against the DNV-RP-F105 response models, and fatigue margins.
- Allowable span length tables by water depth and condition, and a verdict for each surveyed span.
- The scripted model files and a results review call.
- Where it helps, a short animation of the span response for non-specialists.
Codes and standards
- DNV-RP-F105
- DNV-RP-C203
- DNV-RP-F114
- DNV-ST-F101
The assessment is framed against the edition your project specifies.
Related cases
Generic, synthetic studies from the portfolio that show this kind of analysis at work.
Questions clients ask
What sets the allowable span length?
The ratio between the span's natural frequencies and the vortex shedding frequency of the flow around it. Those frequencies depend on span length, support at the shoulders, effective axial force, mass and gap to seabed. The allowable length is the one that keeps fatigue damage within the design life and passes the ultimate checks.
What is the difference between in-line and cross-flow VIV?
In-line VIV moves in the flow direction, starts at lower flow velocities and gives smaller amplitudes. Cross-flow VIV moves perpendicular to the flow, needs higher velocities and gives larger amplitudes. DNV-RP-F105 gives response models for both, and which one governs depends on the current and wave climate.
Can a span that fails the screening be left in place?
Often, yes. Screening criteria are deliberately conservative and are meant to clear spans quickly, not to reject them. A span that fails goes to a full fatigue calculation with site data, and needs support such as grout bags or rock only if that also fails.
Contact
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